The electoral mandate
Unlike the other branches of government the president can claim a popular mandate from the whole American people. (at least at first)
Presidents can be affected by the extent to which they have a mandate to govern. Some presidents are elected on a strong wave of support in which they outline a clear policy vision. Most presidents achieve their most important goals in the first two years of office while their mandate is fresh. This period is referred to as the 'honeymoon'. Presidential success rates typically fall as the term progresses, partly as the president moves further from their original mandate. Obama achieved some of his most important policy goals in his first two years, including the budget stimulus, health care reform and beginning the process of moving troops from Iraq.
Other factors related to the electoral mandate can be just as important. The nature of partisan control is arguably a more important source of presidential power than their public mandate. Armed with a majority in Congress a president is likely to be able to overcome limits to their mandate. It is a party majority that will have a larger impact on their power.
The power of any president is not fixed and the mandate tends to fade — it fluctuates over time depending on many factors. Usually, the power of a president can be compared to that of Congress if ‘power’ is imagined as scales- as the president's power wanes the power of Congress can increase